The transmission of torque, in agriculture and in industry, usually occurs by way of transmission elements, or joints, which are capable of offering a system of protection against overloads, in order to protect the transmission from structural breakages that can go so far as to compromise the safety of the operator.
In agriculture, bolt torque limiters are known which do not allow automatic re-engagement of the transmission, although they do enable synchronized repositioning. Friction disk torque limiters are also known which enable automatic, but not synchronized, re-engagement of the transmission, as is the case with conventional torque limiters using pins.
In the industry, more widely known are torque limiters using cams to disengage, and torque limiters using balls, which enable automatic re-engagement and, if possible, synchronized re-engagement as well.
In torque limiters with pins, an external rotatable element, which is connected to the source of the motor torque, transmits the motion to a driven hub by way of a plurality of pins. On the heads of the pins a contoured profile is defined, which engages in adapted recesses which are provided in the inner surface of the external rotatable element. The head of each pin is pressed against the recesses thanks to a plurality of springs.
The transmission of the torque occurs thanks to the tangential force that develops between the profile defined by the recesses in the external rotatable element and the heads of the pins.
When the value of the moment of resistance of the driven hub increases, a radial thrust is generated on the pin which defeats the resisting force generated by the springs, releasing the head of the pin from the corresponding recess. The head thus rubs against the inner surface of the internal rotatable element, and the value of the torque transmitted decreases sharply to then increase when the pins re-engage in the recesses which follow immediately.
Transmission of the torque during the overload phase is not therefore completely cancelled, since the system allows a “repeating” re-engagement of the torque, effectively perpetuating the overload.
Such conventional torque limiters with pins have, however, the following drawbacks:                correct operation of the protection system is not guaranteed at high speeds, for example higher than 700 rpm;        during overload conditions there are strong vibrations on the transmission;        there is a strong dependency of torque peaks on the elasticity of the system into which the limiter is introduced; often torque peaks occur after the triggering torque which have a value higher than the rating of the limiter;        there are considerable axial encumbrances, in particular for torque limiters at torque values higher than 1000 Nm.        
In conventional torque limiters with cam-operated disengagement, the driving hub transmits the motion to the driven body, which is usually provided with flanged geometry or forked, by way of a series of pins the heads of which engage with adapted seats which are provided in the driven body, by way of recesses which are generally trapezoidal in profile. The pins are free to move radially in cylindrical seats which are provided in the driving hub, into which low-friction bushes are always inserted in order to facilitate the sliding of the pin.
The pin has, in its lower part, a contoured base with a portion that protrudes, engaging between two cams which in turn are preloaded under pressure against each other, with helical or Belleville springs, and which have a contoured profile with two working angles.
The normal operation of the limiter is based on a balancing of two radial forces applied on the pin: a first force generated by the moment of resistance applied to the driven body and a second force generated by the contoured profile of the cams which are subjected to the thrust of the springs. These two working angles of the profile of the cam make it possible to have different values of such second radial force in the configuration of normal operation, with transmission of torque, with respect to the condition where motion is disengaged. In fact when the pin passes completely, with its contoured base, between the cams, the driving hub is free to rotate with respect to the driven body, without transmitting torque. In such configuration, the value of the working angle is such as to generate a low radial force, which is not capable of making the pin re-enter its seat in the driven body unless there is a strong reduction in speed.
One of the main drawbacks of torque limiters with cam-operated disengagement is the necessity to decelerate the rotation in order to make the pin re-enter its seat. Furthermore such torque limiters with cam-operated disengagement are particularly subject to wear of the moveable parts and their operation is ensured up to rotation speeds of 700 rpm.
In conventional torque limiters using balls, the torque is transmitted by means of a plurality of balls which are accommodated in seats provided both in a motor driving disk, which is connected to a driving element, and in a sliding disk, which faces the motor driving disk and is adapted to transmit the torque to a driven element. The balls are kept in their seats by a spring which is adjustable according to the cut-in torque value required, or alternatively by way of pneumatic or hydraulic systems.
In the event of an overload, the balls, defeating the resisting force exerted by the spring, exit from their seats, interrupting the transmission of torque. The movement of the sliding disk is monitored in order to actuate a switch or a safety sensor that is capable of stopping the kinematic chain within a time period that depends on the inertia of the system, and is capable of emitting an alarm signal.
Depending on the configuration and the arrangement of the balls and the corresponding seats between the motor driving disk and the sliding disk, these torque limiters using balls allow manual rearming, or automatic rearming, in a random position (not synchronized) or in a preset position (synchronized).
Such conventional torque limiters with balls have, however, the following drawbacks:                as long as the overload condition lasts, the balls are subjected to maximal axial load forces, which thus limit the rolling thereof and generate hazardous rubbing which causes wear and raising of the working temperatures;        it is necessary to provide a motion arrest sensor for the overload phase, in order to prevent excessive wear which would compromise the functionality of the system, with consequent increase in the complexity and cost of the device;        in order to minimize high values of tangential forces acting on the balls, and excessive stresses on the respective accommodation seats, such torque limiters using balls have considerable radial dimensions, with consequent increase in encumbrances and in rotating inertia values.        